The present invention relates to a process for gas phase polymerization in a fluidized-bed reactor.
It is known to polymerize one or more monomers in the gas phase at a pressure higher than atmospheric pressure in a fluidized-bed reactor where particles of polymer being formed are maintained in the fluidized state by virtue of a reaction gas mixture containing the monomer(s) to be polymerized travelling in a rising stream. The polymer thus manufactured in powder form is generally drained from the reactor in order to keep the bed at a more or less constant volume. A process which is preferred on the industrial scale employs a fluidization grid which distributes the reaction gas mixture through the bed and which acts as a support for the bed in the event of a cut in the flow of the rising gas. The reaction gas mixture leaving at the top of the fluidized-bed reactor is recycled to the base of the latter under the fluidization grid by means of an external circulation conduit provided with a compressor.
The polymerization of the monomers is an exothermic reaction. It is therefore necessary to provide a suitable means for cooling the bed in order to extract the heat of polymerization therefrom. The preferred method for the polymerization of olefins in a fluidized bed consists in cooling the reaction gas mixture below the polymerization temperature, and, when this fluidization gas passes through the bed, this makes it possible to compensate for the excess heat generated by the polymerization. Thus, as it is being returned, the reaction gas mixture is generally cooled with the aid of at least one heat exchanger arranged on the outer circulation conduit so as to remove the heat produced by the polymerization reaction and to keep the polymerization temperature a the desired level.
Attempts have been made, very particularly in recent years, to optimize the gas phase polymerization process so as to increase the output of polymer in existing plants. The thinking has accordingly been in terms of rate of production of polymer, namely in terms of weight yield of polymer produced per unit volume of the reactor and per unit of time (kg/h/m.sup.3). In commercial fluidized-bed reactors of the abovementioned type it is known that the output rate depends directly on the rate of removal of heat generated in the reactor. This rate of removal can be increased, for example by increasing the speed of the fluidizing gas and/or by reducing the temperature of the fluidizing gas and/or by increasing the heat capacity of the fluidizing gas.
For example, in its patent application WO 94/28032 BP Chemicals Limited has proposed a process for has phase polymerization of olefin(s), in which the recycle gas stream is cooled to a sufficient temperature to form a liquid and a gas. By separating the liquid from the gas and by introducing the liquid directly into the fluidized bed it is possible to increase the total quantity of liquid introduced into the fluidized-bed reactor, and this makes it possible to cool the bed better by evaporation and hence to reach higher output efficiency levels.
In general, fluidized-bed reactors according to the present invention can be represented by a first volume, the enclosure (wall) of which consists of at least one surface of revolution generated by the rotation about a vertical axis known as axis of revolution, of a rectilinear and/or curved segment, above which is mounted a second volume, commonly called a disengagement vessel, the enclosure (wall) of which also consists of at least one surface of revolution generated by the rotation, about the same vertical axis known as axis of revolution, of a rectilinear and/or curved segment. According to its definition of disengagement vessel, the orthogonal section of the second volume (at the location situated just above the junction between the two volumes) is higher than the orthogonal section of the first volume (at the location situated at its upper point). Conventional fluidized-bed reactors employed for the gas phase polymerization of olefin(s) usually consist of a cylinder (1) with a vertical axis above which is mounted a disengagement vessel (3) in accordance with FIG. 1, which shows diagrammatically a preferred apparatus for the gas phase polymerization according to the present invention.
The known essential function of the disengagement vessel is to slow down the rising gas stream which, after having passed through the fluidized bed, can entrain relatively large quantities of solid particles. As a result, most of the entrained solid particles return directly into the fluidized bed. Only the finest particles can be entrained out of the rector.
In principle, the fluidized bed could occupy all of the cylindrical part of the reactor, a part which rises over a height H starting from the base of the fluidized bed, which generally coincides with the fluidization grid (4). In practice the fluidized bed generally occupies only a portion of the cylindrical part of the fluidized-bed reactor, with the result that the real height of the fluidized bed (h) is equivalent to 0.95.times.H, preferably 0.90.times.H, and in particular 0.85.times.H. This height limit of the fluidized bed has been dictated by the person skilled in the art in order to avoid excessive entrainment of polymer particles out of the reactor. Studies of fluidization have shown the formation of bubbles within the fluidized bed. Coalescence of the bubbles takes place as they rise within the bed until they burst when they reach the upper part of the fluidized bed. This bursting considerably accelerates the entrainment of the particles out of the reactor. All this has therefore naturally led the person skilled in the art to limit the height of the fluidized bed in a practical manner during the polymerization.
Within the context of research related to the increase in the output efficiency of its industrial plants for gas phase polymerization of olefins, the applicants have succeeded, despite existing prejudices, in developing a simple and reliable process which makes it possible to increase considerably the output of polymers. In addition, the applicants have completely unexpectedly discovered that the use of its new process offers many advantages as disclosed in the description which follows.
The present invention consists therefore of a process for gas phase polymerization in a fluidized-bed reactor consisting of a first volume, the enclosure (wall) of which consists of at least one surface of revolution generated by the rotation, about a vertical axis known as axis of revolution, of a rectilinear and/or curved segment, above which is mounted a second volume, commonly called a disengagement vessel, adjoining the first volume, the enclosure (wall) of which also consists of at least one surface of revolution generated by the rotation, about the same vertical axis known as axis of revolution, of a rectilinear and/or curved segment, characterized in that the fluidized bed occupies at least all of the first volume of the reactor. Thus, according to the present invention the height of the fluidized bed (h) is at least equal to the height H of the polymerization reactor. The fluidized bed preferably occupies at least partially the second volume known as a disengagement vessel.
Unexpectedly, the applicants have has discovered that the process according to the present invention does not in any way lead to excessive entrainment of polymer particles out of the reactor. Although not wishing to be bound to the following explanation, the applicants think that this finding originates, on the one hand, from the fact that the particles undergo a deceleration when they reach the disengagement vessel and, on the other hand, because the bubbles are limited and/or reduced in size when they enter the disengagement vessel.